This invention relates to x-ray systems employing a detector and more specifically relates to monitoring and correcting such systems.
The introduction of new digital x-ray detectors presents a problem with respect to lost image information at locations containing defects within the detector. Defects are typically caused by bad pixel elements, which are pixel elements that are either not responding electrically or have a behavior that is statistically different from their surrounding pixel elements.
During calibration and setup of the digital detector methods exist to identify the location of such defective pixel elements. This is typically done by analyzing flat field images, which contain no structure, and searching for outlier pixel elements. These flat field images are obtained in one of two modes (1) xe2x80x9cdarkxe2x80x9d or offset frames in which the imager is read without using x-rays, and (2) xe2x80x9clightxe2x80x9d or x-ray frames, in which an x-ray signal is applied prior to the read-out. Pixel elements identified during calibration are typically stored in a bad pixel map. Different methods exist to correct pixel elements identified in a bad pixel map prior to image display.
Under certain conditions defective pixel elements, which were not identified during the initial calibration may develop over time in the amorphous silicon detector. The present invention addresses the foregoing problems and provides a solution. In one embodiment, pixel element behavior is monitored, and the bad pixel map may be updated.
An amorphous silicon digital x-ray detector consists of an array of pixel elements (e.g. 2048xc3x972048). Each element consists of a photodiode and an associated field effect transistor (FET). For the purpose of x-ray imaging a scintillator is coupled to the array to convert incident x-rays to light. The light produced by the scintillator is converted to electric charge and stored in the diodes. The charge is read out by activating the FETs associated with each diode.
Algorithms have been designed to analyze images acquired with a flat panel x-ray imager and detect bad pixel elements. These are pixel elements that do not respond electrically or have a behavior that is statistically different from their surrounding pixel elements. Often, this can be related to defects in the FET or diode structure in the array.
The result of these algorithms is to create an image map (bad pixel map) containing the locations of the defective pixel elements within the detector array. Patents in this area include: U.S. Pat. No. 5,657,400, which is directed to automatic identification and correction of bad pixels in a large area solid-state x-ray detector; U.S. Pat. No. 5,854,655, which is directed to a defective pixel detecting circuit of a solid state image pick-up device capable of detecting defective pixels; U.S. Pat. No. 5,272,536, which is directed to a dark current and defective pixel correction apparatus; U.S. Pat. No. 5,047,863, which is directed to a defect correction apparatus for solid state imaging devices including inoperative pixel detection; and U.S. Pat. No. 4,996,413, which is directed to reading data from an image detector.
In addition to detecting the defective pixel elements, some of these patents also describe methods of correcting the pixel prior to image display. Most correction methods rely on replacing the defective pixel with the value of its neighboring pixel or a linear combination of these. In addition some more advanced correction methods based upon the underlying image structure have been proposed in U.S. application Ser. No. 09/474,715, entitled xe2x80x9cCorrection Of Defective Pixels In A Detectorxe2x80x9d filed Dec. 29, 1999 under docket no. 15-XZ-4974 in the names of Aufrichtig, Xue and Kump and U.S. application Ser. No. 09/474,498, entitled xe2x80x9cCorrection Of Defective Pixels In A Detector Using Temporal Gradientsxe2x80x9d filed Dec. 29, 1999 under docket no. 15-XZ-5428 in the names of Aufrichtig, Xue and Kump.
The preferred embodiment is useful in an x-ray imaging system comprising a source of x-rays and a digital detector comprising pixel elements arranged in rows and columns for creating data suitable for generating an x-ray image of a portion of a patient. In such an environment, the detector may be monitored by calibrating the detector during a calibration phase of operation, powering the detector during use phases of operation occurring at different times, energizing the source of x-rays in an exposure mode of operation during the use phases of operation so that x-rays are transmitted to the detector, and inhibiting the source of x-rays in a dark mode of operation during the use phases of operation. The data created by the pixel elements is read and analyzed. The pixel elements are identified which correspond to data indicating defective pixel elements during the calibration phase of operation and during a predetermined portion of a plurality of the use phases of operation.
By using the foregoing techniques, x-ray detectors may be monitored with a degree of ease and accuracy previously unattainable.